Eliminate ALL breads, dairy, and meats other than turkey or chicken. (meats eaten twice a week) Eat fruits, nuts, veggies (raw as much as possible) and atleast two quarts of water a day after one week use a laxitive and cleanse your system out. If your over 40 yrs old get a colonoscopy to secure your health ststus and use their flush system to clean yourself out.
Start walking atleast a mile a day increasing the time to complete every day!
If your able to exercise do sit ups, pushups, chinups and any other "easy" exercise to get you going!! start out doing 15 a day and increase 5 more ever day until your up to 150!! Replace the walk with a jog after 2 weeks and increase the distance using 1/8th of a mile every other day!

Point is start SLOW, use your own body weight for strength and indurance and keep adding as you go along!! It will help after a month to pick a sport to use to increase yor fitness, Boxing is an excellant endeavor! Ray.